This invention relates to the field of foamed plastics, especially foamed liquid crystal polymer sheets or films.
Foamed plastics are well-known. They are particularly useful where light weight or high strength-to-weight ratios are needed. Uses include construction materials, transportation vehicles, industrial equipment, business machines, insulation, and the like. However, many plastics suffer a great reduction in desirable mechanical properties when foamed.
Liquid crystal polymer ("LCP") foams are known in the art. In addition to the advantages inherent in foamed plastics generally, foamed LCP is beneficial for applications requiring LCP properties, e.g. high tensile strength, high temperature performance, chemical resistance and impact strength, but low anisotropy or orientation. U.S. Pat. Nos. 4,429,060 and 4,429,061 describe such foams; the disclosures in these patents are herein incorporated by reference.
U.S. Pat. No. 4,429,060 describes high performance foams comprising at least about 10% of recurring 6-oxy-2-naphthoyl moieties. These lightweight foams are said to have a good strength-to-weight ratio, high thermal stability and excellent solvent resistance. The foams have less anisotropy than unfoamed LCP. These foams may be made in any conventional manner, such as extrusion or injection molding. A blowing agent is needed to cause foaming; suitable conventional blowing agents include 5-phenyltetrazole and trihydrazine triazine.
U.S. Pat. No. 4,429,061 is directed to heat treated LCP foams having a high degree of rigidity and strength. The core of the LCP foams have highly oriented cell walls, and the properties of these cells are enhanced by heat treatment. These foamed LCP's have a low degree of anisotropy compared to unfoamed LCP.
Although the foamed LCP's known in the art have many advantages, their cell structure is not small enough or uniform enough for use as a flexible sheet or film. Their large, non-uniform cells can produce holes in the sheet, a poor surface, brittleness, and poor, non-uniform flexibility.
Unfoamed LCP can be used to produce a uniform, high quality sheet, but these sheets will have a high degree of stiffness. Furthermore, they will suffer from surface delamination and fracture under relatively small radius bends. Consequently, unfoamed LCP is unsuitable for many applications requiring relatively thick sheets. Unfoamed LCP also does not have the light weight, insulative properties and cost advantages of foamed sheet.